Changes induced color formation

Explanations for the phenomenon offered in the literature are largely unsatisfactory. McKay (80) observed that MB solutions turned red only in the presence of strongly basic amines (pKjj < 5) or in other bases (ethoxide, hydroxide), but that they remained blue in weakly basic solvents such as pyridine or quinoline. McKay suggested that Lewis-acid (dye)/base (amine) complexes might be formed, but rejected this explanation in favor of color changes induced by tight ion pair formation between the dyes and the ionic bases such as ethoxide. 

The radiation-induced color changes in inorganic materials (Ref 145) led to a comprehensive study by Rosenwasser, Dreyfus and Levy (Ref 148) on Na azide, which turns to brownish yellow when subjected to radiation. Subsequently, when mechanically deformed crystals of Na and K azide were irradiated with 107R gamma radiation, Dreyfus and Levy (Ref 69) observed the formation of pyramidal etch pits which occurred mainly in regions where imperfections were located at the surface. These were also evident in ammonium perchlorate crystals (Ref 255)... 

An unintentional gas-induced color change to meat occurs when it is exposed to sufficiently high levels of nitrogen dioxide (NOj) such as during incomplete combustion of gases in gas ovens. The surface of the meat becomes pink, much like the characteristic pink of cured meat, due to the formation of nitrosylhemochrome (Cornforth et al., 1998). 

On adding an excess of sodium azide to neutral or slightly acid solutions of cobalt salts, a violet color appears which is due to complex cobalt -azide anions. In contact with air an oxidation to anionic azide-complexes of tervalent cobalt takes place, shown by a change of color from violet to yellow. This very slow autoxidation is enormously accelerated by sulfurous acid or sulfite ions. Probably the autoxidation of sulfite to sulfate induces the oxidation of the complex bounded cobalt. However, the color change violet yellow is not sufficiently sensitive to serve as a test for this induction effect and therefore, for sulfite. It is better to identify the tervalent cobalt through a color reaction with an acetic acid solution of o-tolidine (formation of a blue quinoidal oxidation product of the base). Compare page 209. 

Many biochemical reactions can be induced by temperature increase in foods Maillard reactions, vitamin degradation, fat oxidation, denaturation of thermally unstable proteins (resulting in variation of solubility or of the germinating power of grains, for example), enzyme reactions (which can either be promoted or inhibited), and so on. Some of these biochemical reactions generate components suitable, for example, for their sensory properties (flavor development) others may be more or less undesirable for nutritional or potential toxicity reasons (vitamin losses, changes in color, taste or aroma, formation of toxic compounds). All the reactions are linked to the simultaneous evolution of product composition, temperature and water content (or chemical potential, or water activity), these factors varying diflferently from one point to another, from the center to the surface of the products. 

Figure 6. Enzymes act as recycling catalysts in biochemical reactions. A substrate molecule binds (reversible) to the active site of an enzyme, forming an enzyme substrate complex. Upon binding, a series of conformational changes is induced that strengthens the binding (corresponding to the induced fit model of Koshland [148]) and leads to the formation of an enzyme product complex. To complete the cycle, the product is released, allowing the enzyme to bind further substrate molecules. (Adapted from Ref. 1). See color insert.

A single DBP droplet is positioned in the vicinity of the microelectrode by the laser trapping technique, and the droplet-microelectrode (edge-to-edge) distance (L) is controlled arbitrarily in micrometer dimension. Knowing the oxidation potential of PPD in the water phase to be 30 mV, PPD is oxidized by a potential step method (100 mV) to induce the dye formation reaction. The anodic current relevant to oxidation of PPD reaches a steady-state value within a short electrolytic time (t) because of cylindrical diffusion of PPD to the microelectrode. The dye formation in the droplet can be easily confirmed by the color change from transparent to cyan or yellow. The dye formation reaction in a single microdroplet could be... 

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